Hydraulic circuit for actuation of an earthmoving scraper ejector



June 30', 1964- Av J, HE|N ETAL= 3,138,884

HYDRAULIC CIRCUIT FOR AC'IUWI ION. OF AN EARTHMQV'ING- SCRAPER EJECTORK Filed: March 15 1962. 4 Sheets-Sheet I INVENTORS. ALLYN JHEm ATTORNEYS June 30.- 1964 J" N ETAL, 3,138,884

HYDRAULIC CIRCUIT FOR; AC'TUATION OF AN EARTHMOVING SCR'APER EJECTOR Filed March 15, 1962 4 Sheets-Sheet 2 RESERVOIR EJECTOR MHL II P2 INVENTORS. ALLYN JHEIN BY JOHN A.JUNCI\ A T ORN EYS June 30. 1964 A. J. HEIN ETAL 3,138,884

HYDRAULIC CIRCUIT FOR ACTUATION OF AN EARTHMOVING SCRAPER EZJECTOR Filed March 15, 1962 4 Sheets-Sheet 3 RESERVOIR U INVENTORS. 3 ALLYN J. Ham

\9 BY JOHN A. JUNCK ATTORNEYS June 30. 1964 A. J. HEIN ETAL 3,138,884

HYDRAULIC CIRCUIT FOR ACTUATION OF AN EARTHMOVING SCRAPER EJECTOR Filed March 15, 1962 4 Sheets-Sheet 4 f \ZO INVENTORS.

ALLYN J. HEIN A BY JOHN AJUNCK A TORNEYS United States Patent 3,138,884 HYDRAULIC CIRCUIT FOR ACTUATION OF AN EARTHMOVING SCRAPER EJECTOR Allyn J. Hein and John A. Junck, Joliet, 111., assignors to Caterpillar Tractor Co., Peoria, 111., a corporation of California Filed Mar. 15, 1962, Ser. No. 182,154

6 Claims. (Cl. 37-129) This invention relates to earthmoving scrapers of the kind in which various adjustable elements of the scraper are actuated hydraulically, usually by means of double acting hydraulic jacks and pertains particularly to the hydraulic circuit which controls the operation of the ejector of the scraper. I

The present application is a continuation in part of our co-pending application Serial No. 156,706, now abandoned.

In many large scrapers earth is ejected from the scraper bowl forwardly through its open front end by an ejector which normally closes the rear end of the bowl but is mounted for forward ejecting movement under in fluence of a hydraulic jack. Since scraper bowls have a capacity of many tons of earth considerable force is required to effect such ejecting action. Heavy sticky types of earth such as clay require greater force for ejection than light, dry or granular materials and it is therefore desirable that the lighter materials be ejected at a greater speed than the heavy materials utilizing the power available and increasing the overall speed and efliciency of an earthmoving operation. It is also desirable that a twospeed operation of an ejector be made automatically responsive to the force or inertia which opposes ejection. This enables a two-speed ejector to start against a heavy load in a low speed high force condition and change to a higher speed as the inertia of the load is overcome and as the weight of the load is reduced after partial ejection.

Many ejector circuits are controlled by a sliding spool type hydraulic valve movable in opposite directions from a central neutral position to an ejecting position and a return position. Resilient centering means normally holds the valve in neutral and it must be manually held in its other positions. It is desirable that the ejector control valve be automatically maintained in its return position until full return of the ejector has been accomplished to free the hands of the operator who is occupied in steering and actuating other controls as in returning from a fill area to perform another loading cycle.

It is therefore the object of the present invention to provide a hydraulic circuit and controls for a scraper ejector which include the above mentioned desirable features and which is positive and efficient in operation. Further and more specific objects and advantages of the invention and the manner in which it is carried into practice are made apparent in the following specification by reference to'the accompanying drawings.

In the drawings:

FIG. 1 is a view in side elevation of a tractor trailer combination illustrating the location of the components of the hydraulic circuit of the presentinvention;

FIG. 2 is a schematic view of the entire hydraulic circuit illustrating a two-speed valve in association with the ejector portion of the circuit;

FIG. 3 is a schematic view illustrating portions of the ejector circuit only and showing a modified form of twospeed valve in which actuation of the valve is automatically controlled;

FIG. 4 is an enlarged fragmentary detail of a portion of a control valve associated with the ejector circuit including means employed for holding the control valve in its ejector return position;

FIG. 5 is a fragmentary sectional view of a portion of the same control valve taken on line V-V of FIG. 4 showing other details of the holding mechanism; and

FIG. 6 is a fragmentary section taken on line VIVI of FIG. 3.

In FIG. 1, a conventional two-wheel scraper is illustrated at 10 as drawn by a tractor 11 through a hitch or draft connection generally indicated at 12. The main body or bowl of the scraper may be raised and lowered about its pivotal connection with its wheels 13. Draft arms 14 pivoted to opposite sides of the scraper bowl, as by connections one of which is shown at 15, enable raising and lowering of the scraper bowl by means of a pair of jacks, one of which is shown at 16. A pivoted apron 17 is adapted to be raised and lowered for opening and closing the forward end of the bowl by means of a jack 18, lever 18b and link 18c, and an ejector 19 is moved forwardly to discharge the contents of the bowl through its forward end by an ejector jack 20, all in a well known manner. A control valve assembly 22 for controlling the flow of fluid to and from the several jacks is disposed on the tractor as are also control levers, one shown at 23, positioned adjacent the operators station.

Generally speaking, the hydraulic circuits for operating the adjustable elements of the scraper are the same as those disclosed in our assignees co-pending application entitled Hydraulic Circuit for Tractor Drawn Scrapers and the Like, filed November 24, 1961, Serial No. 154,790 and will be briefly described herein for the purpose of showing the environment of the present invention.

The control valve assembly 22 is schematically shown in FIG. 2 as comprising a single housing which is suitably bored for the reception of three sliding type valve spools 16a, 18a and 29a for controlling respectively the jacks 16, 18 and 20. Each of the spools is formed at one end for connection to control levers and is fitted at its opposite end with a centering spring assembly shown at 26, 28 and 30. The function of such assemblies is well knownand only that pertaining to the ejector control is shown in detail to be hereinafter described.

A pump 35 delivers fluid under pressure from a reservoir 36 to an inlet passage 37 in the valve housing which is divided to direct fluid selectively toward either one end or the other of the valve spool 16a depending upon its position of adjustment. Similar inlet passages 38 and '39 communicate with each other and with the first passage 37 so that with the valve spools in their neutral positions as shown, there is a constant flow of fluid under pressure from the pump through the passages 37, 38 and 39 and a discharge passage 40 and thence back to the reservoir through a return line 41. A relief valve 42 in the pressure line from the pump also has its discharge side connected with the return line 41.

Each of the jacks or sets of jacks has what may be termed a high pressure and a low presseure end because the work of moving an implement part in one direction, as when it is being raised or moving earth, is usually greater than that of moving it in the other direction. Movement of any one of the spools to the right connects it with the high pressure end of its associated jack. For

example, the spool 16a upon movement toward the right opens communication with the inlet chamber 37 containing fluid under pressure from the pump and directs ,the fluid into a line 46 to the rod ends of the jacks 16 for raising the bowl. The fluid first passes through valves 16b associated with the jacks 16 in a manner and for a purpose fully disclosed in our assignees ,co-pending application entitled Hydraulic Circuit for Actuation of an Earthmoving Scraper Bow Serial No. 154,634, filed November 24, 1961. Movement of spool 18a to the right similarlyconnects the inlet passage 38 thereof with a line 48 connecting with the head end of the jack'18 through a valve mechanism 64 described in the above mentioned co-pending application. Upon movement of the spool 20a to the right, fluid under pressure from inlet 39 thereof is directed through a line 50 to the head end of the ejector jack 20. For convenience in following these circuits, the drawings identify the jacks as well as the spools with the part of the implement with which they are associated.

The opposite or low pressure ends of the jacks are all connected with a common manifold 52 which, as shown in the drawings, communicates with each of the three bores which contain the valve spools and is opened by rightward movement of any spool into communication with a discharge manifold 53 also common to all three spools and communicating with the reservoir through the line 41. Consequently when fluid is directed under pressure to the high pressure side of any of the jacks, fluid on the low pressure side is returned to the reservoir. The common manifold 52 is connected with the jacks through a common line 54 with a branch 55 to the low pressure or head ends of the bowl jack 16, a branch 56 to the rod end of the ejector jack 20 and a branch 57 to the rod end of the apron jack 18 through the valve mechanism 64.

Movement of the jacks in the direction opposite that described above is accomplished by movement of any one of the valve spools in the opposite direction or to the left which communicates high pressure through the actuated spool to the manifold 52 and thence to all of the jacks through line 54 and its branches 55, 56 and 57. The valve spool which has been actuated to the left also opens communication to a discharge passage 60 which is common to all of the spools and similar to the discharge passage 53. The discharge passage 60 permits return of fluid from the jack being actuated through the passage 40 and line 41 to the reservoir. Under the condition just described where a single valve spool is actuated and directs fluid to all three jacks, only the selected jack is moved because return flow from the other jacks is blocked by their respective spools which have remained in their neutral position.

The present invention provides means for causing the ejector circuit to operate at either one of two speeds at the selection of the operator. This is accomplished by what will be termed a manual two-speed ejector valve generally illustrated at 65 in FIG. 2. The valve is shown in its high speed position wherein fluid under pressure through line 50 is acting upon the head end of the jack while fluid returning from the rod end of the jack through a line 66 is communicated, by reason of the configuration of a valve spool 67, to a line 68 which also communicates with the head end of the jack. Thus the return fluid from the jack is added to the fluid under pressure rather than returning to the reservoir and causes the jack to actuate at higher speed though with somewhat reduced force.

If the operator anticipates work in heavy or sticky material requiring greater ejecting force, the position of the valve spool 67 is altered by actuation of a lever 70 having a cam 71 thereon to move the spool 67 in opposition to a spring 72 and thus block communication between the lines 66 and 68 and establish communication between the lines 66 and 56 which latter is as previously described a return line for exhausting fluid from the rod end of the jack to the reservoir. In the high speed position of the valve shown it is impossible to return the ejector because the spool 67 is blocking communication between lines 56 and 66. Consequently a port 74 is provided in the spool to permit fluid under pressure in line 56 to enter a chamber 75 in which the cam 71 is disposed and move the spool against action of the spring 72 temporarily to form a connection between lines 66 and 56 and admit fluid to the rod end of the jack for returning the ejector. Movement of the spool against the force of spring 72 either manually or by pressure in the chamber 75 is permitted by a bleed passage 76 which communicates with the spring chamber to exhaust any fluid which may have leaked past the spool and become entrapped in the chamber. In order to prevent the same pressure which flows through passage 74 into chamber 75 from passing through bleed passage 76 into the spring chamber and balancing the pressures on the spool a small check valve 78 is disposed in a passage which communicates between the bleed passage 76 and a side of the spool which communicates with the line 68.

FIG. 3 shows a modification of the two-speed valve 65 wherein the control lever 70 is omitted and valve means are provided to effect change of speeds automatically when the force required to move the ejector changes. In FIG. 3, the valve 65a is similar in operation to that shown at 65 in FIG. 2 and the same reference characters followed by the letter a are employed to designate parts which are similar in construction and function. The valve spool 67a is normally urged to its low speed position by a spring 72a now located at the upper end of the spool as viewed in FIG. 3 to provide communication between lines 66 and 56 as described in connection with FIG. 2. It is desirable that spool 67a be shifted to its high speed position upon introduction of fluid pressure to the head end of the ejector jack 26 through the line 68. In order to accomplish this, pressure in line 68 is communicated through a pasage 80 to the lower end of a reciprocable plunger 81, the opposite end of which bears against the spool 67a to move it upward in opposition to spring 72a. This is effective to block communication between lines 66 and 56 and direct fluid displaced from the rod end of jack 20 to its head end to supplement the pump volume and provide high speed ejection.

In order to provide automatic two-speed ejection, it is necessary that a means be provided to return spool 67a to its normal low speed position when the pressure in line 68 exceeds a predetermined point. For this purpose, a load responsive valve generally indicated at 82 is employed to selectively provide or interrupt communication of pressure to the chamber of spring 72a in response to loads which are reflected by pressure build-up in the head end of the jack and line 68. Pressure in the line 68 is communicated through the valve 65a, in its high speed position, through a passage 83 and connecting passages 84 and 85 in the load responsive valve to the end of a reciprocable plunger 86, the opposite end of which bears against a spool 87 held in the normal position shown by a spring 88. During the ejection of sticky or heavy material tending to cause a build-up of pressure sufficient to overcome the force of the spring 88, the plunger 86 moves spool 87 to the right as shown and high pressure in the passage 83 is communicated through a passage 89 to radial passages 90 in the spool 87, through an axial passage 91 therein and other radial passages 92 to a passage 93 communicating with the end of the spool 67a. Although the pressures acting on the end of plunger 81 and directed to the upper end of spool 67a are equal, due to the area dilferential of these two surfaces and the supplementing force of spring 72a, spool 67a is moved downward to its high force low speed position as described in connection to FIG. 2. Spool 670: will remain in this position until the force required for advancing the ejector is reduced as for example, when the load is nearly discharged from the bowl. At this time spring 88 returns the spool 87 to the position shown. A return path for fluid contained at the end or" the spool 67a while it is held in its high force position is provided by the passage 93, radial passages 92, the axial bore 91 and other radial passages 94 which communicate through passages 95 with the chamber of spring 88 and thence through a passage 96 which communicates with the line 56. At this time the pressure acting on plunger 81 overcomes the force of spring 72:: to shift spool 67a to its high speed position.

When spool 67a is in a high speed position to interconnect the opposite ends of jack 20, the effective working area of its piston 20a is reduced to an area equal to the cross-sectional area of its piston rod 2011. This requires a substantially higher pressure in the head end of the jack than when the entire area of the left face of piston 20a (as viewed in FIG. 3) is effective. For this reason it is desirable that a means be provided to maintain spool 87 in the low speed position until pressure required to advance the ejector reduces an amount greater than the pressure change in the circuit due to a shift to the high speed condition. In order to accomplish this as the pressure acting on plunger 86 becomes great enough to overcome spring 88 and move spool 87 to the right as previously described, the pressure in passage 89 is also directed through an annular groove 87a in spool 87 and a radial clearance provided between a flange 87b on spool 87 and its cooperating bore in the valve body to an area indicated 87c at the left end of the spool. Since the area of the end of spool 87 is substantially larger than the areaof plunger 86, spool 87 will be held in its low speed position until the pressure in the head end of jack 20 has reduced the above described amount. At this time the force of spring 88 will overcome the pressure in area 870 to return spool 87 to the high speed position where it will be uninfluenced by the pressure increase in the circuit due to the change in effective working area of piston 20a.

As spool 67a moves upward to block communication between lines 66 and 56 and provides communication between lines 66 and 68, it is necessary to insure that the fluid being displaced from the rod end of the jack is never completely blocked to permit continuous movement of the ejector. For this purpose a land 97 of spool 67a is of such a size and positioned in relation to the port of line 66 in a manner such that at least partial communication is provided between lines 66 and 68 before the communication between lines 66 and 56 is completely blocked. Due to the fact, however, that relatively high pressure is present in line 68, this partial interconnection of the ports of lines 56, 66 and 68 permits a surge of pressure to flow from line 68 through the port of line 66 into line 56 from where it is directed through passages 96 and 95, radial passages 94 andtaxial passage 91. In order to prevent .this pressure surge from acting on the left end of spool 87 to return it to the position providing communication between line 68 and a chamber of spring 72a as previously described, a small check valve 98 is provided in axial passage 91 intermediate radial passages 90 and 92.

As spool 76a shifts from the high speed to a low speed position due to the sudden change in velocity of the ejector, there is the tendency for the soil being ejected to momentarily leave the ejector, thus resulting in a substantial reduction in the pressure in the head end of the jack and line 68. This reduction in pressure permits spool 87 to return to the position shown to communicate the chamber of spring 72a with the reservoir 36 as previously described and permit spool 67a to return to the high speed position. In order to prevent malfunction of the valve due to this reduction in pressure, communication between lines 66 and 56 through spool 67a is provided through a pluarity of metering slots 130 which are of a size to retrict the fluid returning to the reservoir from the rod end of jack 20. This maintains sufficient back pressure in the rod end of the jack to prevent the excessive reduction in pressure due to sudden change in the velocity of the ejector. This valve has a bleed passage 76a and a check Valve 78a functioning in the manner and for the purpose of the passages 76 and valve 78 of FIG. 2.

It is necessary that spool 67a remain in its normal low speed position when fluid is directed to the rod end of jack 20 to return the ejector to its rearward position in the scraper bowl. For this purpose, a check valve 131 is located in passage 76a of spool 67a to prevent fluid under pressure in line 56 from entering the chamber below the spool 67a and moving it to its high speed position to block connnunication between lines 56 and 66. In order to further insure that spool 67a remtains in its low speed position during this return movement of the ejector, the fluid pressure in line 56 is directed to the chamber of spring 72a through passage 96 and load responsive valve 82.

Since communication between lines 56 and 66 is of a size to restrict the volume of fluid displaced from the rod end of jack 20 during ejection of material contained in the bowl and the volume of fluid being directed to the rod end of the jack through lines 56 and 66 is greater in volume than the displaced fluid, the restriction in the valve 65a would create an excessive back pressure in line 56 thus resulting in wasted horse power and heat generation in the hydraulic circuit. In order to provide unrestricted flow of fluid to the rod end of jack 20, a by-pass line 132 is provided between the lines 56 and 66. A check valve 133 is located in the bypass line and is effective to permit free flow from line 56 to line 66 and to block communication in the opposite direction so that fluid returning to the reservoir 36 must pass through the restriction of slots 130 in spool 67a as previously described.

Another desirable feature of the present invention is that it provides means for holding the ejector control spool 20a in the ejector return position until the ejector completes its return stroke, thus freeing the operator for attention to other controls. Referring first to FIG. 2 it will be recalled that during the return stroke of the ejector fluid from the head end of jack 20 is being returned through line 50 under pressure toward the reservoir. This pressure results from returnmovement of the piston in jack .20 and resistance created by transmission through the various conduits which return fluid to the reservoir. Part of this pressure is transmitted to the spring chamber of the centering spring assembly 30 by means of a passage indicated in broken lines at in FIG. 2 and more clear- 1y illustrated in FIG. 4 which is an enlarged section taken through the center of this passage as viewed'at right angles to FIG. 2. Fluid under pressure in passage 100 opens a spring pressed check valve 101 admitting fluid through a passage 102 in the valve housing and a slot 103 in the spool to the spring chamber and maintaining the spool in its leftward position which is that shown in FIG. 4. The pressure in the spring chamber of centering assembly 30 is desirably limited to a value which can be overcome by the operator through manual control linkage in the event that he wishes to interrupt return movement of the ejector. This is accomplished by.a check valve assembly shown at 105 in FIG. 5 wherein a spring biased ball prevents passage of fluid from the spring chamber through passages 106 which communicate with slots 107 in the spool 20a and through a passage 108 to a line 109 (see also FIG. 2) which leads to the reservoir. Thus if the operator through manual controls urges the spool 20a toward its neutral position with force suflicient to unseat the check valve 105 (preferably about 65 psi) the spool can be adjusted from its ejector return position'of FIG. 4 to the neutral position of FIG. 5. If the return movement of the ejector is not interrupted by the operator the spool 20a will automatically return to its neutral position When the ejector completes its travel. Since the piston of jack 20 reaches the end of its stroke when the ejector is fully returned pressure in the line 50 fails thus permitting the check valve 101 to close. As the centering spring in the assembly 30 urges the spool toward its neutral position fluid in the spring chamber bleeds off, first through the slot 103, passage 102 and a small orifice 111 in the check valve 101. Since the spool 20a is moving toward the right from the position of FIG. 4 the slot 103 is closed during the initial movement. However, just before closing of the slot 103 the slot 107 registers with the spring chamber and permits continued bleeding through orifices 112 in a spring seat 115 therein which communicate fluid through slot 107 and passage 108 to the line 109 communicating with the reservoir.

At the completion of the operation just described the centering spring chamber remains filled with fluid which might produce a fluid lock preventing the spool 20a from moving toward the right which is necessary to extend the jack 20 .for ejecting material from the bowl. This fluid lock is relieved by a bleed port 114 normally closed by the spring seat in which the orifices 112 are formed. However, initial movement of the spool 20a toward the right removes the spring seat 115 from its position obstructing the bleed port 114 and relieving the hydraulic lock.

Holding the ejector spool in the return position as described above depends upon pressure created by return movement of the cylinder in the ejector jack 20. Therefore should either of the spools 16a or 18a be actuated to adjust the position of the bowl jacks 16 or apron jack 18 normal flow of pump pressure through area 39 (see FIG. 2) would, because the spools are in series, be interrupted cutting off pressure to the ejector jack. Hence the piston of the ejector jack would stop, causing failure of return pressure to line 50 and permitting spool 20a to return to neutral position before the ejector has been fully returned. This is prevented by borrowing pressure from either of the spools 16a or 18a which is actuated at this time. The means for accomplishing this is shown in FIGS. 2, 3 and 6 wherein conduits 116 and 117 are shown as communicating between the bores in which spools 16a and 18a slide and, through a junction box 119, with a conduit 120 leading to the spring chamber of check valve 101. Thus pressure is supplied through passage 102 and slot 103 to the centering spring chamber to substitute for the pressure lost upon interruption of movement of the ejector jack. The means for communicating pressure from the spool bores to the lines 116 and 117 is identical in both cases and is shown in FIG. 6 where the spool 18a is shown in its neutral position. A passage 122 communicating with conduit 117 is closed by the spool in this position. This passage is opened by movement of the spool toward the right when it is uncovered by a land thereon. Upon movement to the left pressure is communicated to the passage 122 first through passages 123 then through passages 124 and finally through notches 125 formed in the edge of the land. The passages 123 and 124 register with grooves 126 and 127 respectively which circumscribe the land and maintain its normal ability to seal while a small portion of the pressure in the valve chamber is utilized as described for holding the ejector valve spool 20a in its return position.

We claim:

1. In combination with an earthmoving scraper having a bowl and an ejector moveable through the bowl for ejecting its contents, a circuit for hydraulic fluid under pressure, and a jack in said circuit for moving the ejector, means to direct fluid to one end of the jack, a first means to return fluid from the opposite end of the jack to a source, a second means to direct fluid from said opposite end to said first end to speed the operation of the jack, a valve to direct fluid selectively through said first and second means, and means responsive to pressure of fluid directed to said one end of the jack to actuate said valve to direct fluid from said opposite end of the jack selectively to the first end or the source.

2. The combination of claim 1 in which said valve includes a sliding spool, resilient means holding the spool in one selective position and means for directing pressure from said one end of the jack to slide the spool to the other position in opposition to said resilient means.

. 3. In an earthmoving scraper having a bowl with an apron and an ejector and having hydraulically actuated means for raising and lowering the bowl and for actuating the apron and ejector including sliding spool valves for selectively directing pressure to said hydraulic means, means operable upon movement of the ejector control spool to ejector return position for directing fluid under pressure to one end of said spool for retaining it in said position, and means operable upon complete return of the ejector for relieving said fluid pressure.

4. The combination of claim 3 including relief valve means to limit pressure at the end of said spool to a value which can be overcome by manual operation of the spool.

5. In an earthmoving scraper having a bowl an apron and an ejector therefor and having hydraulic means for adjusting the bowl, apron and ejector, separate spool valves'in a common'circuit for controlling flow of hydraulic fluid under pressure to and from said several hydraulic means, said spool valves being arranged in series whereby fluid pressure to one valve may be interrupted by actuation of the others, means actuated by fluid pressure in the ejector circuit to hold the ejector spool in an ejector return position and means effective upon actuation of the bowl or apron spools during such holding to supply pressure from the bowl or apron circuit to said holding means.

6. In an earthmoving scraper having a bowl, an ejector therein and hydraulically actuated means for advancing the ejector to eject the contents of the bowl including a source of fluid under pressure and a double acting hydraulic jack, a circuit for communicating fluid to one end of the jack, a return line from the opposite end of the jack, a line between said opposite end and the first named end and valve means operable to direct fluid selectively from the opposite end to the return or to said first end to provide two operating speeds, said valve means being actuated by pressure resulting from resistance to movement of the ejector.

References Cited in the file of this patent UNITED STATES PATENTS 

1. IN COMBINATION WITH AN EARTHMOVING SCRAPER HAVING A BOWL AND AN EJECTOR MOVEABLE THROUGH THE BOWL FOR EJECTING ITS CONTENTS, A CIRCUIT FOR HYDRAULIC FLUID UNDER PRESSURE, AND A JACK IN SAID CIRCUIT FOR MOVING THE EJECTOR, MEANS TO DIRECT FLUID TO ONE END OF THE JACK, A FIRST MEANS TO RETURN FLUID FROM THE OPPOSITE END OF THE JACK TO A SOURCE, A SECOND MEANS TO DIRECT FLUID FROM SAID OPPOSITE END TO SAID FIRST END TO SPEED THE OPERATION OF THE JACK, A VALVE TO DIRECT FLUID SELECTIVELY THROUGH SAID FIRST AND SECOND MEANS, AND MEANS RESPONSIVE TO PRESSURE OF FLUID DIRECTED TO SAID ONE END OF THE JACK TO ACTUATE SAID VALVE TO DIRECT FLUID FROM SAID OPPOSITE END OF THE JACK SELECTIVELY TO THE FIRST END OR THE SOURCE. 